This invention relates generally to liquid crystal cells and more specifically to liquid crystal display (LCD) cells having a flexible substrate formed of materials such as plastics, polymers and the like and to electronic apparati having such a liquid crystal element installed therein.
There has been an increasing demand for plastic film panel (PFP) liquid crystal cells having a flexible substrate formed from materials such as plastics, rather than glass, to be used as display elements for pagers, portable telephones and the like. Such liquid crystal cells are generally light, are not easily broken, and are therefore more transportable. Liquid crystal cells having a flexible substrate have attracted attention as display elements and are sought to replace cells using glass substrates since increased portability is attained due to the features of lightness and unbreakability of the liquid crystal display element. Thus, more reliable liquid crystal cells having flexible substrates are strongly desired. As a result, there has been a great demand that the display mode be changed from a twist nematic (TN) type to a supertwist nematic (STN) type having greater display quality.
Although liquid crystal display cells having flexible substrates have the above-described beneficial features, they also have problems compared with liquid crystal cells which use normal, inorganic glass as a substrate. The problems of liquid crystal cells having flexible substances are as follows: (1) since heat resistance temperature is low, for example, about 150.degree. C., a low temperature manufacturing process must be employed. Therefore, manufacturing conditions and selection of materials different from glass cells are required; (2) since organic substrates typically have inferior resistance to chemicals or solvents compared to glass substrates, the surface of the substrate, display electrodes and the like are more likely to be damaged during stress involving the application of an acid or alkali material such as steps in which the electrode pattern is formed; and (3) since organic materials such as PFP have inferior resistance to the atmosphere, it is easy for gas such as oxygen or other vapors to enter the liquid crystal layer, causing gas bubbles to be generated.
Therefore, the use of flexible substrates for liquid crystal display cells imposes significant limitations on the manufacturing process and materials which constitute the liquid crystal element. Furthermore, reliability matching that of a glass cell has not yet been achieved.
Ordinarily, reliable liquid crystal materials commonly sealed into glass liquid crystal cells have a skeleton such as phenylcyclohexane or biphenyl (hereinafter referred to as "PCH-type liquid crystal material" and "biphenyl-type liquid crystal material", respectively). PCH-type liquid crystal material and biphenyl-type liquid crystal material were included in liquid crystal cells having a glass substrate. An environmental resistance test, including exposure to high temperatures, was conducted and no problems occurred in the liquid crystal cell. However, when these same liquid crystal materials were used in a liquid crystal cell having flexible organic substrates, it was found that the amount of electric current greatly increased over time, and orientation irregularities were observed, thereby deteriorating the quality of the display considerably.
Accordingly, it is desirable to provide a liquid crystal display device having a flexible substrate, which device has a high quality of display as well as exhibiting lightness, unbreakability and portability.